cerastium tomentosum (snow-in-summer)

Here's another tough ground-cover for sunny, dry conditions.  In fact, I've heard you can grow it under a spruce tree, although I haven't tested that myself. Be careful where you plant it, it spreads.

This one is outside a basement window.  We had our basement renovated last fall and the guys passed a lot of stuff through this window and trampled all over this plant.  It doesn't seem to have minded at all!!